A conventional optical head apparatus comprises: a diffraction grating for producing three beams for detecting tracking errors based on a light beam emitted from a semiconductor laser; and a beam splitter which reflects the three beams to allow the three beams to enter an optical disc and, in addition, permits a reflected light beam received from the optical disk through the objective lens to pass therethrough and to enter a photodetector.
In this optical head apparatus, three beams produced in the diffraction grating are applied to the optical disc, and the diffracting position of the diffraction grating is set so that the +first-order light beam and the -first-order light beam are located respectively before and after the direction of advance of tracking pit, and tracking errors are detected based on the difference between the +first-order light beam and the -first-order light beam. Further, a light beam reflected from the optical disc is passed through a beam aplitter to produce astigmatism, and focusing errors are detected based on this.
Another conventional optical head apparatus is provided with a detecting optical system using, for example, a moire fringe. This optical head apparatus is described in Japanese Patent Laid-Open Nos. 168213/1990 and 137130/1990.
The conventional optical head apparatus, however, has the following disadvantages.
The first problem is as follows. There is a demand for an increase in capacity of information signals recorded on an optical disc, and, in fact, the increase in density has been advanced in the art. Regarding the detection of errors by the positional control of the focusing beam, the magnification has been determined in such a manner that the degree of a variation in spot on the optical disc relative to the degree of a variation on the detecting optical system does not lead to an increase in the optical head apparatus. The sole method for realizing a high definition detection sensitivity corresponding to an increase in information density is to increase the magnification and to increase the length of the optical path within the optical head apparatus. A design of a high-density error detection system, which can cope with the increase in information density was impossible from the viewpoint of the limitation of the geometry of the optical head apparatus.
The reason for this is as follows. The sole method for as detecting the displacement on the information face of the optical disc is to direct a design requiring an increase in optical magnification on the objective lens side and the detecting optical system side. Increasing the optical magnification, however, has resulted in increased length of the optical path within the optical head apparatus, leading to an increase in size of the optical head apparatus. Increasing the magification means that the distance between the semiconductor laser and the objective leas is increased. Due to the properties of the semiconductor laser an a divergent beam, the efficiency for light utilization is lowered resulting in deteriorated reproduced signals. On the other hand, satisfying the efficiency for light utilization recuires increasing the effective diameter of the objective lens, leading to an increase in the size of the optical head apparatus.
The second problem is as follows. The optical construction for detecting optical error signals is one of the factors which increase the size of the optical head. This is an obstacle to a reduction in size. In the conventional focusing error detection system, an astigmatism system using a half mirror has been mainly used. In this system, a restriction on the design of the optical head necessitates increasing the length of the optical path by using a concave lens. In the error detection of an optical disc having a multi-value structure for realizing further increased information density, the geometry of pits is reduced according to the multi-value, posing a problem that the magnification for the detection of the conventional optical head apparatus does not provide satisfactory resolution.